1. Field of the Invention
The present invention relates generally to power supply circuitry suitable for supplying driving power to a portable optical mark reader, such as a bar code scanner. More particularly, the invention relates to power supply circuitry which can minimize changes in voltages converted from voltages of a direct current power supply and supplied to a load even though the state of the load is considerably changed.
2. Description of the Related Art
Hitherto, for facilitating the operability of a portable optical mark reader, such as a bar code scanner, to enhance its portability, built-in batteries, such as miniature secondary cells and dry cells, are used as a power supply source. Further, in order to avoid enlargement of such a portable device, the number of built-in secondary cells and dry cells are reduced to a minimal level, and a direct-current-to-direct-current converter (DC-to-DC converter) compensates for shortage of cells and is provided for power supply circuitry to generate a voltage required for driving a load.
FIG. 4 is a circuit diagram illustrating an example of the configuration of conventional power supply circuitry for supplying power to a portable optical mark reader, such as a bar code scanner. The power supply circuitry is, as illustrated in FIG. 4, adapted to convert an output voltage of a built-in power supply source 41, such as a miniature secondary cell or dry cell, and to supply driving power to a load 45. The power supply circuitry has a DC-to-DC up-converter 42 for raising an output voltage of the built-in power source 41, a large-capacity shunt capacitor 43, and a ripple filter 44 for eliminating ripple components. The DC-to-DC up-converter 42 is formed of a series inductor 46, a field effect transistor (FET) 47, a Schottky barrier diode (SBD) 48, a self-excited oscillator circuit 49, an operational amplifier (OP) 50, a reference voltage source 51, and resistor dividers 52 and 53. The ripple filter 44 includes a series inductor 54 and a shunt capacitor 55. Moreover, the output terminal of the DC-to-DC up-converter 42, the input terminal of the ripple filter 42, and one end of the shunt capacitor 43 are connected to an output line 56.
A portable optical mark reader, such as a bar code scanner, driven by the aforedescribed power supply circuitry, enters a heavy-load mode and more specifically, the resistance of the optical mark reader is small as viewed from the power supply, when marks are read or the read marks are transmitted in remote control signals, such as light signals. In contrast, the optical mark reader enters a light-load mode, and more specifically, the resistance of the mark reader is large as viewed from the power supply, when in a standby position in which mark reading or remote-control signal transmitting is not performed.
FIG. 5 illustrates the operational states of the known power supply circuitry shown in FIG. 4 when the individual factors of the load 45 are significantly changed: FIG. 5a illustrates a change in the resistance (impedance) of the load 45; FIG. 5b illustrates a change in a load current flowing in the load 45; FIG. 5c illustrates a change in a load voltage applied to the load 45; and FIG. 5d is a change in an output current of the DC-to-DC up-converter 42.
The conventional power supply circuitry operates in the following manner when the resistance of the load 45 is large, i.e., under light-load conditions.
In the DC-to-DC up-converter 42, the self-excited oscillator circuit 49 generates a square-wave signal having a predetermined duty cycle and applies it to the gate of the FET 47. When this square-wave signal is at a positive level, the FET 47 is actuated to allow an inductor current I.sub.L to flow from the built-in power supply source 41 via the series inductor 46 and the FET 47, thereby charging the series inductor 46 with energy proportional to the inductor current I.sub.L. When the square-wave signal is changed to a negative level, the FET 47 is turned off to discontinue the flow of the inductor current I.sub.L. Instead, the charging energy supplied to the series inductor 46 is emitted to the output line 56 via the SBD 48 so as to generate a raised voltage corresponding to the charging energy in the output line 56. The increased voltage is then divided at the resistor dividers 52 and 53 and supplied to the non-inverting input terminal of the operational amplifier 50. In the amplifier 50, the voltage is compared with a reference voltage generated by the reference voltage source 51.
When the raised voltage is within tolerance of a prescribed voltage, the divided voltage fed to the operational amplifier 50 is substantially equal to the reference voltage, and the output of the operational amplifier 50 is a first voltage indicating that there is no difference between the voltages supplied to the two input terminals of the amplifier 50. The self-excited oscillator circuit 49 maintains the above-described predetermined duty cycle of the square-wave signal while the first voltage is being fed to the circuit 49.
If the increased voltage is lower than the tolerance of the prescribed voltage, the divided voltage supplied to the operational amplifier 50 is lower than the reference voltage to create a difference therebetween. The operational amplifier 50 then generates a second voltage obtained by adding a positive voltage equivalent to a difference between the divided voltage and the reference voltage to the first voltage. In response to this second voltage, the oscillator circuit 49 increases the predetermined duty cycle of the square-wave signal. This lengthens the actuation duration and shortens the inactuation duration of the FET 47, which further increases the energy charged to the series inductor 46 and elevates the energy output from the series inductor 46 accordingly. Control is exerted in this manner so that the increased voltage can be contained within the tolerance of the predetermined voltage.
In contrast, if the raised voltage is higher than the tolerance of the predetermined voltage, a divided voltage fed to the operational amplifier 50 is equally higher than the reference voltage to create a difference in a manner similar to the case where the increased voltage is lower than the reference voltage. Thus, the operational amplifier 50 generates a third voltage obtained by adding a negative voltage equivalent to a difference between the divided voltage and the reference voltage to the first voltage. In response to this third voltage, the oscillator circuit 49 decreases the predetermined duty cycle of the square-wave signal. This shortens the actuation duration and lengthens the inactuation duration of the FET 47, which further reduces the energy charged to the series inductor 46 and lowers the energy output from the inductor 46 accordingly. Control is exerted in this manner so that the increased voltage can be contained within the tolerance of the prescribed voltage, in a manner similar to the case where the increased voltage is lower than the prescribed voltage.
During the above-described operation, the increased voltage of the output line 56 is charged by the large-capacitance shunt capacitor 43 and is substantially averaged. Subsequently, the ripple components contained in the increased voltage are removed by the ripple filter 44, and the resulting voltage is then supplied to the load 45.
An explanation will now be given with reference to FIG. 5 of the operation of the known power supply circuitry when the resistance of the load 45 decreases, i.e., under heavy-load conditions.
During a first period from time t.sub.0 to t.sub.1, a portable optical mark reader, such as a bar code scanner, is in a standby mode, in which the resistance of the load 45 is large, as shown in FIG. 5a, i.e., under light-load conditions. In this state, a load current is small (minimum), as illustrated in FIG. 5b, while a load voltage is large (maximum), as shown in FIG. 5c. The output current of the DC-to-DC up-converter 42 is small (minimum), as illustrated in FIG. 5d.
At time t.sub.1, the portable optical mark reader is caused to transition from the standby mode to a mark reading mode or a remote-control signal transmitting mode. At this time, the resistance of the load 45 is sharply reduced, as shown in FIG. 5a, and the mode is abruptly changed from a light-load mode to a heavy-load mode. The load current is sharply changed from the minimum to a high level (maximum), as illustrated in FIG. 5b, while the load voltage starts to gently decrease from the maximum, as shown in FIG. 5c. The output current of the DC-to-DC up-converter 42 starts to gently increase from the minimum, as illustrated in FIG. 5d. The major portion of the load current at time t.sub.1 is discharging current from the shunt capacitor 43.
During a second period from time t.sub.1 to t.sub.2, the portable optical mark reader is set in the mark reading mode or in the remote-control signal transmitting mode. The resistance of the load 45 is small, as illustrated in FIG. 5a, under heavy-load conditions, while the load current progressively decreases from the high level (maximum), as shown in FIG. 5b. The load voltage progressively decreases, as illustrated in FIG. 5c, and the output current of the DC-to-DC up-converter 42 slightly increases, as shown in FIG. 5d. The load current during the second period initially maintains a comparatively large value because the major portion of the load current is discharging current from the shunt capacitor 43. However, when this discharging current is progressively decreased, the load current is also progressively lowered because a progressive decrease in the discharge current from the shunt capacitor 43 cannot be completely compensated with an increase in the current output from the DC-to-DC up-converter 42.
At time t.sub.2, the portable optical mark reader is returned to the standby mode from the mark reading mode or the remote-control signal transmitting mode. At this time, the resistance of the load 45 abruptly increases, as illustrated in FIG. 5a, under light-load conditions. In this state, the load current reduces to a low level (minimum), as shown in FIG. 5b, while the load voltage is caused to transition, as illustrated in FIG. 5c, from a low level to a high level. The output current of the DC-to-DC up-converter 42 starts to gently decrease, as shown in FIG. 5d. The load current at time t.sub.2 is part of the output current of the DC-to-DC up-converter 42 because the shunt capacitor 43 has completed discharging.
During a third period from time t.sub.2 to t.sub.3, the portable optical mark reader is maintained in the standby mode. At this time, the resistance of the load 45 is large, as shown in FIG. 5a, under the light-load conditions. In this state, the load current is maintained at a low level (minimum), as illustrated in FIG. 5b, while the load voltage sequentially increases, as shown in FIG. 5c, to finally return to the high level (maximum). The output current of the DC-to-DC up-converter 42 slightly decreases, as illustrated in FIG. 5d, to finally return to the low level (minimum).
In the aforedescribed known power supply circuitry used in a portable optical mark reader, the DC-to-DC up-converter 42 has a small current-supplying capability, as low as approximately 150 mA, because only a miniature secondary cell or a dry cell is used as the built-in power source. In contrast, the load current intermittently flows with extreme changes (for example, a duty cycle of 1/20 or smaller) by switching between the heavy-load condition and the light-load condition. Accordingly, the average current is 150 mA or smaller, which can be contained within the current-supplying capability of the DC-to-DC up-converter 42. However, the peak load current flowing in a short period of time (for example, 1 millisecond to 100 milliseconds) is approximately 1 A, which far exceeds the current-supplying capability of the DC-to-DC up-converter 42. The abovedescribed operation can be typically observed in power supply circuitry used in a portable optical mark reader as a load. In the aforedescribed known power supply circuitry, when the resistance of the load 45 is considerably changed, the load voltage sharply drops, thereby causing erroneous operation.
Hitherto, for preventing such erroneous operation, means for substantially maintaining the load voltage is provided in response to a significant change in the resistance of the load 45. Among the above known means, the most simple is to increase the capacitance of the shunt capacitor 43, for example, to raise it to approximately 80000 .mu.F. This is much larger than the averaged load current, which disadvantageously enlarges the volume of the power supply circuitry, thereby increasing the size and weight of the overall portable optical mark reader having the built-in power supply circuitry. Additionally, the manufacturing cost is increased.
Another means for substantially preserving the load voltage is to charge a capacitor under light-load conditions and to discharge the capacitor under heavy-load conditions so as to compensate for a decrease in the load voltage caused by heavy-load conditions with a discharging current generated by the capacitor. An example of such means is disclosed in Japanese Unexamined Patent Publication No. 4-251532.
Although this means can compensate for a decrease in the load voltage under heavy-load conditions, a charging current generated by a capacitor is primarily used to offset the load voltage. Accordingly, a considerably large-capacitance capacitor is required, thus enlarging the volume of the power supply circuitry. This further increases the size and weight of the overall portable optical mark reader having the built-in power supply circuitry. The manufacturing cost is also increased.